1. Field of the Invention
The present invention relates to a slit exposure optical apparatus used for, for example, an electrophotographic copying machine, and more particularly, to a focus adjusting mechanism thereof.
2. Description of the Prior Art
As is known, in an electrophotographic copying machine of the type in which an original table is fixed, two movable tables are used to maintain constant a light path length of an original image incident upon a projecting lens. More particularly, FIG. 3 illustrates an exposure optical apparatus having the construction as described, and a first movable table 4 which moves along guide rails (not shown) and guide rods 3 on opposite sides of an optical system box 1 comprises a slit plate 6 for scanning an original (not shown) illuminated by an illuminating lamp 5 and a first mirror 7 which reflects an incident light from a slit 6a of the slit plate 6. A second movable table 8 that may be moved in a direction similar to that of the first movable table 4 comprises a second mirror 9 receiving a reflected light from the first mirror 7 and a third miror 10, the third mirror 10 causing the reflected light from the second mirror 9 to incident upon a projecting lens 11. Accordingly, in the first movable table 4 and second movable table 8 constructed as described above, the second movable table 8 has to be moved in the same direction at a speed of one half (1/2) with respect to the first movable table 4 to maintain constant an incident light path length relative to the projecting lens 11. In the past, however, to attain such synchronous drive, a wire pulley device as shown is used between a fixed portion of the device and the first and second movable tables 4, 8. This wire puley device will be further described. The wire pulley device comprises a pair of fixed pulleys 12, 13 opposedly provided in a moving direction of the first and second movable tables 4, 8, two relay pulleys 14, 15 positioned in the middle between the fixed pulleys 12, 13, two movable pulleys 16, 17 supported on ends of the second movable table 8, and a driving drum 18 rotated by a motor not shown. A wire 20 one end of which is fixed to a stay 19 of the optical system box 1 is stretched over the movable pulley 16, thereafter fixed to an end 4a of the first movable table 4, reversed at the fixed pulley 12, passes through the relay pulley 14, the driving drum 18, the relay pulley 15 and the fixed pulley 13 in said order, stretched over the movable pulley 17, and the other end of the wire is fixed to a stay 21.
In the above-mentioned exposure optical apparatus, a distance L from the first movable table 4 to the second movable table 8 is adjusted so as to have a fixed incident light path length to adjust focus. However, in the construction as described above, a fixing screw 22 of the end 4a of the first movable table which fixes the wire 20 is untightened to change the relation between the first movable table 4 and the wire 20. In this case, there is only one method which pulls the wire fixed to the screw by a turning amount of the screw using an adjusting screw. Therefore, in the past, an original table glass (not shown) or other cover is removed to expose the first movable table end 4a, thereby adjusting the focussing operation. However, such a partial disassembling operation involves cumbersome operations in terms of manufacturing process, maintenance and management, and the focussing time becomes extended.